FIG. 3 is a circuit diagram illustrating a structure of a conventional battery charging circuit (see, for example, Non-Patent Document 1). FIG. 4A illustrates current waveforms of an electric power generator. FIG. 4B illustrates voltage waveforms of the electric power generator. Hereinafter, an operation of the conventional battery charging circuit is explained with reference to FIGS. 3, 4A, and 4B.
Hereinafter, a circuit operation, when a voltage VB of a battery B is low (the battery B is not sufficiently charged), specifically, when the voltage VB is lower than a predetermined voltage determined based on a breakdown voltage of a zener diode ZD1, is explained. In a voltage detection circuit 4, a reverse current (from a cathode of the battery B to an anode of the battery B) does not flow in the zener diode ZD1. For this reason, a current does not flow between a base and an emitter of a transistor Q1. Therefore, the transistor Q1 changes to the off-state. In this case, in a switch control circuit 5, a current does not flow between a base and an emitter of a transistor Q2. Therefore, the transistor Q2 also changes to the off-state. Since a gate current of each of thyristors S1 to S3 is zero in a switching circuit 3, the switching circuit 3 switches to the off-state (in which a current does not flow from an anode to a cathode). Accordingly, a three-phase AC voltage outputted from an electric power generator 1 is rectified by each of diodes D1 to D6, and thus the battery B is charged with the rectified voltage as a charge voltage (FIG. 4B).
Hereinafter, a circuit operation, when the battery B is sufficiently charged and the voltage VB thereof becomes higher than the predetermined voltage, is explained. In the voltage detection circuit 4, a reverse current from the battery B to the zener diode ZD1 flows. Accordingly, the transistor Q1 changes to the on-state. Then, a current flows between the base and the emitter of a transistor Q2 in the switch control circuit 5, and therefore the transistor Q2 also changes to the on-state. Since a gate current flows in each of the thyristors S1 to S3 through the transistor Q2 and the resistors R1 to R3, each of the thyristors S1 to S3 changes to the on-state (in which the current flows from the anode to the cathode). Therefore, the three phases of the electric power generator 1 are short-circuited through the diodes D4 to D6, respectively. Consequently, the battery B changes to a non-charged state.
[Non-Patent Document 1] Japanese Unexamined Patent First Publication No. H10-70851